Matte glazes achieve their surface through controlled devitrification — partial crystallization during cooling that scatters light.
Understanding This Defect
When the matte effect is patchy (some areas matte, others glossy, or the matte texture varies across the tile surface), the devitrification process is not uniform. Common flux-related causes: insufficient CaO for crystal nucleation (the calcium-alumina-silicate crystals that create the matte effect need adequate CaO to nucleate); uncontrolled cooling rate (too fast = no crystallization, too slow = over-crystallization with rough texture); and inconsistent flux chemistry between batches. Our GP20 grade is specifically formulated for matte glazes: its high CaO (5.34%) provides the calcium nucleation source that promotes uniform crystal growth across the entire surface. Combined with high total alkali (15.63%) for complete initial melting, GP20 ensures the glaze first melts fully at peak temperature, then crystallizes uniformly during the controlled cooling phase. Use GP20 at 15-25% of glaze formulation, cool at 3-5 degrees C per minute through the 1100-900 degrees C crystallization range, and add 2-4% zinc oxide for enhanced matte effect..
Solution & Prevention
